PURPOSE: To enable an unskilled operator to easily estimate a fish amount, by a method wherein the fish amount, corresponding to an optional part of an image picture, is computed, and the result is indicated togetherwith a boundary line of the applicable water area.

CONSTITUTION: A plural number of time points from each start position of a main scanning signal generator and a subscanning signal generator of a raster scanning indicator in a television set and are selected to form a boundary line which can optionally change a position and an area in a picture, and a point crossing the raster scanning line and said boundary line is indicated in a particular color. A sum of products of a level and an echo of fishes in a picture surrounded with the boundary line, and an integration vaue (echo aggregate value) of a fish echo concentration correspondling to said sum of products are found, and a correction, which corresponds to picture-setting conditions, such as a mean depth at the water area surrounded with the boudary line, a range width/picture width, water speed of a transducer/image feeding speed, etc., is made to the echo aggregate value of them. The fish amount in the water area surrounded with the boundary line is computed to find a numeral value, which can be estimated, for the indication.